1. Field of the Invention
This invention relates to integrated-circuit (IC) chips having semiconductive switches such as might be used in multiplexers to connect any one of a number of inputs to an output. More particularly, this invention relates to so-called fault-protected overvoltage switches.
2. Description of the Prior Art
Fault-protected overvoltage switches have been provided commercially for some time. Such switches typically are arranged to open automatically if an excessive voltage is applied to the switch input (or the output in some cases), thereby preventing the overvoltage from reaching and damaging a device to which the switch is connected. Commonly, such switches comprise a number of series-connected MOSFET transistors. For example, they may include an N-channel MOSFET, a P-channel MOSFET, and another N-channel MOSFET, in that order.
Although the available prior art devices function reasonably well, they have a number of disadvantages. One disadvantage is that the circuits used heretofore require ancillary elements including diodes and additional MOSFETs to prevent problems such as are caused by reverse currents to the voltage supplies of the part. These ancillary elements require extra die area, thus increasing the cost of manufacture. They also degrade the performance of the switch in certain respects. These and still other disadvantages of the prior art have made it desirable to provide an improved switch design to avoid or minimize the problems with the prior art designs.